1. Field of the Invention
Oil and Gas Exploration risk management includes the ability to control subsurface pressures which may be encounter during drilling operation. The primary mechanism utilized by operators to control downhole pressures is the hydrostatic pressure as a result of the drilling fluid contained within the wellbore. The drilling fluid is engineered and formulated to a density that provides a hydrostatic pressure inside of the wellbore that is greater than the formation pressure being drilled. In the majority of drilling operations, the hydrostatic control of wellbore pressure is adequate. However, from time-to-time the operator may encounter a higher than expected formation pressure where there is not adequate hydrostatic pressure to control the wellbore pressure. During these times the operator relies on a series of mechanical controls to stabilize the wellbore and prevent a “Blowout”. A blowout is the uncontrolled release of fluid or gas from the wellbore. This event is extremely dangerous and therefore must be avoided if at all possible. The primary mechanical control device utilized by operators to control wellbore pressure is the Blowout Preventer (BOP) assembly. The BOP assembly consists of multiple sealing and shearing devices that are hydraulically actuated to provide various means of sealing around the drill string or shearing it off entirely thereby completely sealing the wellbore. A hydraulic pressure source and a means of controlling the hydraulic fluid under pressure are required for proper BOP operation.
2. Description of Related Art
Typically hydraulic pressure is provided by utilizing high pressure hydraulic accumulators and a control panel near the drilling platform. These accumulators are typically charged to 3000 PSIG but in some case to a higher pressure. In testing or actual operation a series of hydraulic valves are opened to direct the flow of high pressure hydraulic fluid to the appropriate pressure control device of the BOP assembly. To operate as designed the hydraulic fluid received at the BOP assembly must be at a pressure equal to or greater than the minimum required by the manufacture or governing body. Typical land-based surface BOP systems require a minimum of 1200 psi to operate as designed. In the current state of the art systems the relationship between the hydraulic accumulator pressure and the hydraulic pressure available to operate the BOP is 1 to 1. For example if the accumulator hydraulic pressure is 2,250 PSIG then the hydraulic pressure available to operate the BOP assembly is 2,250 PSIG. As hydraulic fluid is expelled from the accumulators to operate the BOP assembly the hydraulic pressure of the accumulator will decrease. At some point the hydraulic pressure in the accumulator will not be sufficient to operate the BOP assembly. This minimum acceptable hydraulic pressure level is typically set at 1,200 PSIG but may be more or less depending on the actual BOP setup. The rate at which the pressure decreases in an accumulator is proportional to the volume of fluid discharged from the accumulators. This is defined by Boyle's law P1 V1=P2 V2. Applying this to a typical 15 gallon hydraulic accumulator utilize on drilling rig we find:                Volume at 1,000 PSIG per charge level: 15 gallons        Volume at 1,200 PSIG minimum pressure level: 12.5 gallons        Volume at 3,000 PSIG maximum pressure level: 5 gallons        Working volume (12.5-5)=7.5 gallons.        
Working from the example above it is reasonable to expect 7.5 gallons of hydraulic fluid at a minimum of 1,200 PSIG from each accumulator in the accumulator rack. The number of accumulator bottles in a typical accumulator rack vary significantly based on the hydraulic requirements of the various BOP assemblies. For illustrative purposes, a typical 20 tank accumulator rack and a typical surface BOP assembly will be utilized in the subsequent example. Based on the previous calculation above, it is reasonable to expect 150 gallons of hydraulic fluid at a minimum of 1,200 PSIG from the accumulator rack.
Proper BOP operation is critical for safe Oil and Gas Exploration activities. The American Petroleum Institute (API), a widely recognized trade origination, has developed standards related to the manufacturing and testing of BOP assemblies. A typical test of the BOP assembly of this example, in accordance with the guidelines of API 53, would require approximately 75 gallons of hydraulic fluid at a minimum pressure of 1,200 PISG. From the previous example above, it is reasonable to expect 150 gallons of hydraulic fluid at a minimum of 1,200 PSIG from the accumulator rack. Therefore it is reasonable to expect that the accumulator rack could supply sufficient pressurized hydraulic fluid to complete two API 53 tests with each test consuming approximately 75 gallons of pressurized hydraulic fluid. Subsequent to these test it is necessary to recharge the accumulator rack utilizing a hydraulic pump. This pump could be either pneumatic or electric. Additionally these pumps can be utilized as an emergency hydraulic power source for the BOP if the accumulator rack is fully depleted due to an anomaly or unforeseen situation. A disadvantage to this system is the very limited amount of pressurized hydraulic fluid available before recharging is required. Recharging requires a power source which may not be available during an extreme emergency situation. Another disadvantage of this system is the inefficiency associated with the control of hydraulic fluid discharged from the accumulators as it is utilized to operate the BOP system. In a typical BOP operation the initial ram closing cycle does not require 1200 psi. In fact the initial portion of the ram closing cycle can be accomplished with as little as 250 psi. The initial closing cycle may be as much as 75% of the complete closing cycle. The pressure required for the subsequent remaining 25% of the cycle will increase exponentially to approximately 1,200 psi depending on the BOP system and drill string being utilized. The hydraulic energy discharged from the accumulator bank during a closing cycle is equal to the pressure of the accumulator bank and the flow rate of the discharge. The discharge flow rate from the accumulator bank to the BOP system is controlled by a flow control valve. As previously stated the initial pressure of the accumulator bank is approximately 3,000 psi but the first initial 75% of the closing cycle only requires 250 psi. During this phase of the ram closing cycle the flow rate is regulated by the flow control valve. The energy discharged from the accumulator bank during the closing cycle is directly related to the flow and pressure of the hydraulic fluid discharged by the accumulator bank. The energy consumed by the BOP ram is also directly related to the inflow and pressure required to operate the ram. The energy difference or imbalance between that discharged by the accumulator bank and that consumed by the BOP ram is lost as heat at the flow control valve. The energy loss can be substantial. For example, during the very first part of the ram closing cycle the accumulator energy discharge is approximately 15 times greater than that required by the BOP. This can be established by looking at the flow and pressure relationship between of the accumulator rack and the BOP closing ram. The flow rate of the hydraulic fluid discharged at the accumulator rack is equal to the flow rate of the hydraulic fluid consumed by the BOP closing ram. However the pressure discharged at the accumulator rack is initially 3,000 psi but the pressure required at the BOP closing ram is only 250 psi. Therefore, the energy ratio is 12 to 1 (3,000/250). The balance of energy is heat loss from the pressure drop across the control valve. Also note that the accumulator rack is at the highest pressure when the BOP closing rams operating pressure requirement is the lowest. As the accumulator rack pressure decreases linearly with the discharge of hydraulic fluid, the pressure requirement of the BOP ram will increase. As some point near the end of the BOP ram closing cycle the pressure requirement will have increased to approximately 1,200 psi. If the accumulator pressure is less than 1,200 psi it will not be able to fully close the BOP ram. FIG. 7 depicts the energy relationship between the accumulator rack and the BOP ram.
It is much more preferable to more efficiently utilize the stored energy of the accumulator rack to extend its capacity and usefulness to operate the BOP systems. The ideal system would automatically increase or decrease the energy use ratio between the pressure available at the accumulator rack and that required by the BOP closing ram during the entire closing cycle or any other BOP operation.